SRT101 battery question
In 1967 I wanted a SRT101 ans couldn't afford it. I just recently got a buy on one that I couldn't resist ( happens way too often) and i read alot about some of the old mercury battery cameras not needing any modification with modern batteries and I've seen the SRT listed as one of these. Can anyone give me 1st hand experience on this?
While I am not an electrical engineer, I believe that the metering circuit in the Minolta SRT-101 is built around a so-called "bridge circuit" where one key design feature is battery voltage independence. Proper exposure is indicated when the circuit becomes "balanced" against a reference voltage.
Many cameras employed less expensive inline circuits that were voltage dependent. Mercury batteries worked well in these designs because their lifetime discharge curves were very flat with a steady, predictable voltage. And the circuits were less complex to implement.
Current silver oxide batteries also have very flat discharge curves similar to mercury cells. But they are normally ~1.5-volt designs, whereas the mercury batteries were ~1.35-volts. The silver oxides should, I believe, work just fine in a bridged circuit design, but would require a voltage modification of some sort to work with inline designs. Or an adjustment to the camera's ASA/ISO setting.
There are currently available 1.5-volt silver oxide button cells manufactured in the original PX625 mercury cell form factor. I use these in my little Canonet QL17 GIII, which I've read is an inline design, but in my camera was modified for 1.5-volts.
I also have a lifetime stash of original PX625 mercury cells in deep freeze. These guys have an almost obscene life span. The one currently powering my Yashica MAT-124G TLR meter has been in the camera since January, 2004. It's still going strong.
Last edited by Ken Nadvornick; 03-30-2010 at 12:45 AM. Click to view previous post history.
Reason: Needed justa leeedle bit more...
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I am an electronic engineer and I have a couple of SRTs - Unfortunately I can't remember if they have a bridge circuit or not!
Originally Posted by Ken Nadvornick
It's easy to tell though. If the meter shows correct exposure at a central position and the meter needle also rests at that position when switched off then it's safe to assume that it is a bridge circuit and it will be fine with a 1.5 volt cell. If the correct exposure is made by lining one needle up with the position of the meter needle then it's not a bridge circuit and will be sensitive to voltage changes.
I have a feeling that the SRT is not a bridge circuit though as I seem to recall discussing the addition of a schottky diode into such a camera with someone a while ago.
EDIT: The SRT does not have a bridge circuit so you will need to do one of these things:
1. Use a 1.5 volt cell and compensate by changing the ISO setting.
2. Use a hearing aid battery.
3. Use a modern cell in an adaptor incorporating a schottky diode.
4. Convert the camera by fitting a schottky diode inside.
Here is a PDF showing the conversion: http://www.rokkorfiles.com/conversion.htm
Last edited by Steve Smith; 03-30-2010 at 02:33 AM. Click to view previous post history.
There is an article that I printed off about 5 years ago when I put my second last px625 into my srt, about how to recalibrate it when the mercury stash was gone.
I came across it last night digging for a cyanotpe sensitizer formula.
Recalibration as my article describes involves metering an evenly lit wall/flat surface using a stable flood light, and measuring the light reading with another trusted camera/ light meter.
Then fit the new non-mercury battery, and with the bottom plate of the camera removed, adjust one or two trimmer potentiometers until the srt meters the same as the independently established exposure.
PM me and I can scan the article if you are interested and cannot find the information elsewhere.
my real name, imagine that.
There are two trim resistors if I remember correctly. One is to set the battery check position and the other is to calibrate the actual light meter deviation. Obviously to do it properly, both will need to be set up.
I would try to use as much light as possible to do the light meter calibration. Getting it to about 80% of full scale deflection seems like a reasonable way to do it.
EDIT: The circuit showing the two variable resistors is in the link I attached earlier. Looking at the circuit it would appear that you need to calibrate it as a light meter before you calibrate the battery check. In all probability, if you calibrate the meter side of it, the battery check position should still be o.k.
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One characteristic of mercury batteries was that they output a voltage which did not significantly change over the life of the battery. Alkaline batteries, by contrast, start out producing more voltage than rated, which then declines in a relatively stable curve until they're close to exhaustion, at which point voltage falls off a cliff. The SRT's metering circuit was designed for the stable voltage from a mercury battery. So, while an alkaline batter will work, the meter will not closely match what it should read because the battery voltage will not be what the meter is expecting it to be.
You can either modify your SRT, as above, so that it can use modern alkaline batteries, or you can buy Wein cells. Wein cells are excellent replacements for mercury batteries, although they do not have the life of a mercury battery. To use a Wein cell, you peel a sticker off the battery, which lets air in, which starts the reaction which produces electricity. After about six months or so the cell is exhausted. You can prolong the life by removing the cells from your camera if you don't anticipate using it for a while, and putting the sticker back on (or covering with something like electrical tape.
I have an SRT which has been modified, and another in which I use Wein cells. They seem to work equally well.
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A better option may be to do the modification but use a silver oxide cell.
Originally Posted by mjs
I haven't tried it but I have heard that you can make them last longer by only un-covering one of the air holes rather than taking off the whole sticker. As the meter doesn't need a lot of current, the cell doesn't need so much air.
Originally Posted by mjs
I had the schematic diagram of the srT101 but I can't find it just now. The meter circuit isn't a bridge circuit. It simply connects in series with the battery, a fixed resitor, a trim pot, 2 CDS photocells and the galvanometer. The movement of the galvanometer is totally dependent of the light level. ISO, lens aperture and shutter speed settings are coupled to the mechanical needle. When the 2 needles matched you have correct exposure.
I tested my SRT101 with and external regulated power supply with voltage of 1.35V and 1.5V. The meter reading with 1.5V is less than 1/2 stop higher than that of 1.35V (higher reading means less exposure). With either 1.35V or 1.5V the meter isn't very accurate. While I can adjust the reading via the trim pot the meter movement isn't very linear. If I adjust it so it's correct at EV14 then it would read about EV4 at EV6.
I have the SRT101 for more than 20 years and I never used the built in meter to take pictures. Although I own 2 very good handheld meters rarely ever used them when I used the SRT101. Using color negative films I can't remember when I had bad exposure with the SRT101. My trouble with exposure only started when I got the Nikon F5 in 2002 with its famous matrix meter then I got many underexposed shots. I sent the F5 back to factory under warranty twice to have the meter calibrated and twice it came back with the meter that most of the time underexposes my color negative film.
Here is a replacement battery from our friends at Freestyle:
"I tested my SRT101 with and external regulated power supply with voltage of 1.35V and 1.5V. The meter reading with 1.5V is less than 1/2 stop higher than that of 1.35V (higher reading means less exposure). With either 1.35V or 1.5V the meter isn't very accurate."
Isn't that's just one reason why there is a lot of "slush/latitude" in the various stages built into film photography?
I modified my own SRT-101 by soldering a diode into the circuit; it matches my other cameras very closely using a silver-oxide battery now. Unlike Alkalines, which should never be used, the silver-oxides have a flatter discharge curve like mercury.